1. Field of the Invention
This invention relates to a method of cleaning a surface of a compound semiconductor crystal of Group II-VI elements of Periodic Table, such as ZnSe, ZnSSe, ZnS, ZnMgSe, ZnMgSSe, ZnTe, etc.
2. Description of the Related Art
It has hitherto been reported that ZnSe crystal is etched with an aqueous solution of NaOH or NH.sub.4 OH/H.sub.2 O.sub.2 or etched with a saturated solution of potassium dichromate in sulfuric acid at 90.degree. C. and treated with carbon disulfide, thereby cleaning the crystal surface (Cf. "J. Crystal Growth" 86 (1988), pp 324). Furthermore, it has been reported that ZnSe crystal is etched with a solution of bromine/methanol and subjected to ultrasonic washing in trichloroethylene (Cf. Japanese Patent Laid-Open Publication No. 18200/1984) and that ZnSe crystal is etched with a solution of sulfuric acid/ hydrogen peroxide/water and then cleaned with carbon disulfide to remove Se on the crystal surface (Cf. Japanese Patent Laid-Open Publication No. 109694/1993).
According to the above described cleaning methods, however, there arise disadvantages that the crystal surface is roughened, Se is deposited to be remarkably red, pits occur due to crystal defects, impurities such as Na, etc. in an etching solution remain and a clean, flat and mirror surface cannot be obtained. Thus, etching cannot be carried out in such a depth that a damage layer is removed. Consequently, an epitaxial thin film of high quality cannot be grown on a crystal obtained by the above described surface cleaning method.